A friend of mine gave me some powdered milk before she moved. It was dry packed in mylar bags and was not the kind of powdered milk I usually buy but was instant milk purchased from Sam’s Club. Neither of us knew if it would still be good.
I was pleasantly surprised to find that the milk, dry packed in 2003 (19 years ago), was still good. It was still a good color and it smelled fresh. One key to the success was that both she and I stored it inside an air-conditioned house.
Preserving non-fat dry milk by dry-packing (sealing in a container with an oxygen absorber) works. It keeps it fresh and very usable for a long time especially if stored relatively cool.
Note: One bag was not completely sealed. The milk in it was brown and smelled bad. Make sure you have a good seal on your mylar bags.
